In a small bowl, stir together the 1 cup flour, baking powder, and 1/4 tsp. salt. Set aside. In a large mixer bowl, beat together the butter and sugar on medium speed until light and fluffy. Add in the egg until smoothly incorporated.Combine the half & half and the vanilla. At low speed, add in the flour mixture alternately with the liquid--three parts flour and two parts liquid, beginning and ending with the flour.Pour batter in baking dish and top evenly with blackberries.Topping:In a bowl, mix 1/3 cup sugar, 1/3 cup flour, cinnamon, 4 Tbsp. butter, and 1/8 tsp. salt using a pastry blender or your fingers to make a crumbly topping. Sprinkle evenly over the berries.Place the dish on a baking sheet to prevent any bubbly spills and bake for about 35 to 40 minutes. Check the buckle early on to make sure the top edges aren't over browning. The finished buckle may still look kind of wet in the middle, but that's okay. The sides and bottom of the buckle should be beautifully golden. This is a super moist dessert that's gooier in the middle. Let it cool for a while on a rack.